Monday, May 4, 2015

Magic Tweak

Just a quick post to alert ML users that the latest nightlies incorporate my diffraction corrected depth of field reporting: go to original post

I was pleased to see the latest nightly, as it was based on my coding which I proved by compiling my own version of ML.

The only 'downside' is that the current implementation may appear slightly confusing to some as someone else decided to tweak my version.

The important thing to note is that if the on-screen feedback is in white text, then the depth of field is being correctly reported to account for both defocus blur and diffraction blur. For a full frame camera this is up to about F/16, ie beyond that diffraction gets the better of you and you are advised to not go there. The code automatically accounts for cropped sensors.

If the text goes yellow/orange this is a WARNING that the depth of field reporting can no longer be relied on and the near and far information has little real value. So back off, back to white text.

By switching to the ML menu screen when this occurs, you can ascertain what criterion is being breached, ie either the Airy limit, ie defocus blur is too small, or the diffraction limit has been reached, ie diffraction blur is greater than the total blur, assumed as 29 microns for a full frame and scaled for an APS-C sensor. This can be changed in the code, for example for a more exacting standard.

By noting the near and far values (when white) you can undertake landscape focus stacking, ie ensure each refocused image overlaps the last one, until the far value shows infinity.

Finally, the diffraction assumes a visible band camera. If you have an IR converted camera you will need to change one number in the code. For those that are interested on what a bit of ML code looks like, pop along to view the source.

The ML reporting includes the hyperfocal, which is the point of focus where everything from half that distance to infinity falls within the acceptable depth of field, or acceptable 'out of focusness', as only one point/plane is truly in focus. The hyperfocal is the largest depth of field possible for a given f-number. The hyperfocal point is easily spotted when the far reporting just shows infinity. For insurance it is best to just focus beyond that point.

The near DoF is the nearest distance, relative to the sensor, where objects appear in focus according to a combination of defocus and diffraction blur, taken in quadrature.

The far DoF is the farthest distance, relative to the sensor, where objects appear in focus according to a combination of defocus and diffraction blur, taken in quadrature.